The present invention is generally directed to toner processes, and more specifically, to the preparation of resin particles by emulsion polymerization, and which resin particles can be selected for use in toner aggregation and coalescence processes, reference, for example, U.S. Pat. Nos. 5,344,738; 5,403,693; 5,418,108; and 5,364,729, the disclosures of which are totally incorporated herein by reference. In embodiments, the present invention is directed to the economical in situ chemical preparation of toners without the utilization of the known pulverization and/or classification methods, and wherein toners with an average volume diameter of from about 1 to about 25, and preferably from 1 to about 10 microns, and narrow size distribution can be obtained, the size distribution as measured by GSD being in the range, for example, of about 1.05 to about 1.40, and preferably in the range of 1.05 to 1.3. The resulting toners can be selected for known electrophotographic imaging and printing processes, including color processes, and lithography.
In embodiments, the present invention is directed to emulsion polymerization processes whereby the colloidal properties of the resulting resin particles can be controlled in a manner that the latexes of resin particles may be aggregated and coalesced in the processes described in U.S. Pat. No. 5,403,693, and in similar processes, over a wider range of conditions and, therefore, with improved consistency and reproducibility. Specifically, U.S. Pat. No. 5,403,693 illustrates the addition of extra stabilizer after the formation and before the coalescence, or fusing of the desired aggregates, thereby "freezing" the aggregate size prior to the coalescence step. If no extra stabilizer is added, or if too little extra stabilizer is added, then the aggregates may exhibit an increased tendency to grow further in size during the coalescence step and the GSD of the particle size distribution will tend to increase, whereas if too much extra stabilizer is added, then the aggregates may begin to break apart. Since both the further growth/increase in GSD and the breaking apart of aggregates are generally not desired, there exists a need for a process wherein a limited range of concentrations of extra stabilizer can be used to "freeze" the aggregates without producing either undesirable growth or breakage. For aggregates formed from some latexes, this range is quite narrow, or may even not exist, because the latex properties are not optimized for the aggregation process.
Accordingly, the present invention is directed, in embodiments, to the use during emulsion polymerization of reagents that ensure adequate termination, for example by chain transfer termination, of growing oligomer chains either in the water phase or at the interfaces between the water and particle phases, phases which coexist during emulsion polymerization, to produce latex particles with colloidal properties that are more desirable for aggregation-coalescence processes than the properties of similar latexes made without such reagents. The particle phase refers to the growing particles, which comprise resin polymer, such as poly(styrene-co-butyl acrylate); monomer, such as styrene and butyl acrylate; and other reagents, or components, such as termination transfer agents, surfactants, and polar comonomers. Various effective amounts of termination agents, such as alkyl thiols, can be selected, such as from about 0.0002 moles per 100 grams of monomer to about 0.09 moles per 100 grams of monomer, and preferably from about 0.0005 moles per 100 grams of monomer to about 0.04 moles per 100 grams of monomer. The present invention in embodiments utilizes the above mentioned class of termination agents in the emulsion polymerization step to minimize or eliminate the breakdown of aggregated particles that may occur when practicing the processes disclosed in U.S. Pat. No. 5,403,693 and similar processes, thereby resulting in a superior process wherein the particle size is controlled over a substantially wider range of conditions.
While not being desired to be limited by theory, it is believed that the breaking apart of aggregates occurs when the additional surfactant that is added to "freeze" the aggregate size is able to penetrate between aggregated latex and/or pigment particles, thereby disrupting the attraction between these particles. This penetration can occur when these particles have not approached each other within a sufficiently short distance, which may be as small as 2 to 10 Angstroms, and form a sufficiently strong attraction. Thus, latex particles with colloidal properties which do not allow the particles to aggregate in a sufficiently intimate manner will form aggregates that are susceptible to breaking apart. For example, particles with a large number of charge groups of the same sign chemically bound to their surface (dissociatable polar groups arising from polar comonomers, acidic comonomers such as acrylic acid and methacrylic acid) will not experience as large an attraction to one another, since the charge groups on two neighboring particles will repel one another. These repulsions between the charge groups can render it more difficult for the particles to come into close contact. Even the hydrophilicity of polar but uncharged groups chemically bound to the surface may render it more difficult for the particles to come into close contact. When latexes are synthesized by emulsion polymerization with polar comonomers, an example being polar comonomers with groups that can dissociate to yield charged groups (e.g. acidic or basic comonomers such as acrylic acid), the polar comonomers may react to form polar groups which are chemically bound to the surfaces of particles, depending on the process and whether the disclosed termination agents are absent, which can prevent a sufficiently strong attraction from forming between two aggregating particles. When the attraction is not sufficiently strong, the aggregates formed from such particles will have a greater tendency to break apart. Agents which ensure adequate termination of growing chains either in the water phase or at the interfaces between the water and particle phases can be used in the emulsion polymerization to minimize or eliminate this problem, since such agents can reduce the number of such polar groups that are chemically bound to the surfaces of the resin particles. In this invention, such agents are, therefore, added in large enough amounts to ensure the desired termination, and the processing conditions are also chosen to ensure the desired termination, yielding latex particles with the desired colloidal properties.
While again not being desired to be limited by theory, it is expected that polar comonomers can react to become a part of either (i) species that reside primarily in the water phase, (ii) interfacially active species that adsorb onto the resin particles, or (iii) polymer chains that are incorporated into the bulk of the emulsion particles. In (iii), the polar comonomer units are chemically bound to the particle and will have very limited mobility below the glass transition temperature of the polymer, once the polymerization is completed; furthermore, such units will reside either at the surface or in the interior of the particles. Polar comonomer units which are chemically bound to the surface of the emulsion particles may weaken the aggregation of particles, and the use of termination agents reduces the degree to which such comonomers become chemically bound to the surfaces of the emulsion particles, thereby reducing or eliminating the problems arising from a weakened aggregation. When polar groups are chemically bound to the surfaces of latex particles, they may form a permanent barrier between aggregated latex particles which keeps the latex particles from strong attraction to each other. Conversely, adsorbed, but not chemically bound, interfacially active species containing polar units can change positions along the surface, as well as desorb, in order to allow the formation of closer and stronger attractions between primary particles. In emulsion polymerization, initiation of growing chains occurs primarily in the water phase, and the growing chains add polar comonomer units in relation to their concentration in the water phase. In the absence of termination, these growing chains eventually enter and incorporate into a particle, which may not be optimal for aggregation processes, however, the incorporation can be reduced by ensuring that a sufficient number of the growing chains are terminated either in the water phase or at the interface between the water and particle phases. The resulting molecules will then fall under (i) or (ii), rather than (iii), thus yielding latex particles with more favorable colloidal properties. Termination agents, such as certain chain transfer agents, with sufficient reactivity in the water phase or at the interfaces between the water and particle phases, can be used to effect the desired termination in polymerization processes. In embodiments, the present invention is directed to processes for controlling the colloidal properties of resin particles through the use of termination agents, including certain types of chain transfer agents, that ensure the mode of termination described herein, and in polymerizations with polar comonomers. As illustrated in the Examples hereinafter, some chain transfer agents used to modify the molecular weight of the resin are also effective in causing the mode of termination described above, but other chain transfer agents used to modify the molecular weight of the resin are not effective in this role.
In embodiments, the present invention is directed to synthesizing a latex for use in aggregation/coalescence processes for preparing toner, e.g. processes disclosed in U.S. Pat. No. 5,403,693, wherein a stabilizer is added to a suspension of aggregates prior to heating the aggregates to a sufficiently high temperature to enable fusing, or coalescence, of the aggregates, the action of the stabilizer being the prevention of further growth of the aggregates during the coalescence stage. It is believed that the use of termination agents during preparation of latexes by emulsion polymerization, namely, agents such as certain chain transfer agents which are believed to cause adequate termination either in the aqueous phase or at the interfaces of reacting latex particles during emulsion polymerization with polar comonomers, results in latex particles with improved colloidal properties in that breakup of aggregates during the coalescence stage is minimized or prevented with better control and over a wider range of conditions than is often achieved otherwise.
As an example, the present invention is directed, in embodiments, to an in situ process comprised of (i) first dispersing a pigment, such as SUNSPERSE CYAN.TM. or SUNSPERSE RED.TM., in an aqueous mixture containing a cationic surfactant, such as benzalkonium chloride (SANIZOL B-50.TM.), utilizing a high shearing device, such as an IKA/Brinkmann Polytron, or microfluidizer or sonicator; (ii) thereafter shearing this mixture with a charged latex of suspended resin particles, such as poly(styrene/butylacrylate/acrylic acid), synthesized using a termination agent which ensures adequate termination either in the aqueous phase or at the interfaces of emulsion particles, of particle size ranging from about 0.01 to about 0.5 micron as measured by the Brookhaven nanosizer, in an aqueous surfactant mixture containing an anionic surfactant, such as sodium dodecylbenzene sulfonate, for example NEOGEN R.TM. or NEOGEN SC.TM., and nonionic surfactant, such as alkyl phenoxy poly(ethyleneoxy) ethanol, for example IGEPAL 897.TM. or ANTAROX 897.TM., thereby resulting in a flocculation, or heterocoagulation of the resin particles with the pigment particles; and (iii) further stirring for from about 1 hour to about 24 hours with optional heating at from about 0.degree. to about 25.degree. C. below the resin Tg, which Tg is in the range of between 45.degree. to 90.degree. C. and preferably between about 50 and 80.degree. C., resulting in formation of statically bound aggregates ranging in size of from about 0.5 micron to about 10 microns in volume average diameter size as measured by the Coulter Counter (Microsizer II); and (iv) adding concentrated (from about 5 percent to about 30 percent) aqueous surfactant solution containing an anionic surfactant, such as sodium dodecylbenzene sulfonate, for example NEOGEN R.TM. or NEOGEN SC.TM., or nonionic surfactant, such as alkyl phenoxy poly(ethyleneoxy) ethanol, for example IGEPAL 897.TM. or ANTAROX 897.TM., in controlled amounts to prevent any changes in particle size and in GSD of the size distribution, which can range from about 1.16 to about 1.28, during the heating step, and thereafter, heating to 10.degree. to 50.degree. C. above the resin Tg to provide for particle fusion or coalescence of the polymer and pigment particles; followed by washing with, for example, water to remove surfactants, and drying, whereby toner particles comprised of resin and pigment with various particle size diameters can be obtained, such as from 1 to 12 microns in average volume particle diameter, and preferably in the range of 3 to 9 microns, and wherein the stirring speed in (iii) is reduced in (iv) as illustrated in U.S. Pat. No. 5,403,693. The aforementioned toners are especially useful for the development of colored images with excellent line and solid resolution, and wherein substantially no background deposits are present. This invention is directed in embodiments to the synthesis of latex particles whereby the behavior of the suspended aggregates in the heating, or coalescence step is improved by the use during the emulsion polymerization of termination agents, such as chain transfer agents, which can increase the rate of termination of growing chains in the water phase or at the interfaces between the particles and the water phase. When such reagents are used, the propensity of aggregates to fall apart upon addition of extra surfactant and subsequent heating of the aggregates during (iv) is diminished.
Of importance with respect to the processes of the present invention in embodiments is the combination of (a) adding a termination agent, for example 1-butanethiol, 1-octanethiol or CBr.sub.4, with sufficient reactivity in the water phase or at the interface of the water and particle phases, such as a chain transfer agent with sufficient water solubility or interfacial activity, during formation or growth by emulsion polymerization of resin particles; and (b) controlling the amount of anionic or nonionic surfactant added to already formed aggregates, as disclosed in U.S. Pat. No. 5,403,693, to ensure, for example, that the dispersion of aggregated particles remains stable and thus can be effectively utilized in the coalescence process in a manner which maintains control of particle size and particle size distribution. The addition of the extra portion of anionic or nonionic surfactant prior to coalescence increases the repulsion between the aggregates, thus enhancing the stability of the aggregated system against further increase in aggregate size to such an extent that the aggregates can essentially retain their particle size during the coalescence step. Controlling the amount of added surfactant in (b) can be important, especially for the preparation of small toner composite particles, since one can control particle growth in the aggregation step and retain those particles through the heating stage. These advantages are disclosed in U.S. Pat. No. 5,403,693. Conversely, the aggregates may break apart into smaller entities upon addition of this extra stabilizer and subsequent heating, which is detrimental to the process and product. The tendency of the aggregates to break apart, when it occurs, may be reduced or eliminated by using a sufficient amount of any terminating agent capable of causing adequate termination of growing chains either in the water phase or at the interface between the water and particle phases when preparing the latex by, for example, emulsion polymerization.
There is illustrated in U.S. Patent 4,996, 127 a toner of associated particles of secondary particles comprising primary particles of a polymer having acidic or basic polar groups and a coloring agent. The polymers selected for the toners of this '127 patent can be prepared by an emulsion polymerization method, see for example columns 4 and 5 of this patent. In column 7 of this '127 patent, it is indicated that the toner can be prepared by mixing the required amount of coloring agent and optional charge additive with an emulsion of the polymer having an acidic or basic polar group obtained by emulsion polymerization. Also, note column 9, lines 50 to 55, wherein a polar monomer, such as acrylic acid, in the emulsion resin is necessary, and toner preparation is not obtained without the use, for example, of acrylic acid polar group, see Comparative Example I. Unlike in the present invention, aggregates in the process described by U.S. Pat. No. 4,996,127 continue to increase in size when the temperature of the suspension of aggregates is increased, including when the suspension is heated in order to fuse the aggregates. No method of minimizing or preventing the growth of aggregates prior to fusing, or coalescence is disclosed, nor is a method disclosed for reducing the tendency of aggregates to break apart upon addition of extra stabilizers and subsequent heating, when such a tendency arises. Furthermore, the use of termination agents during emulsion polymerization to advantageously alter the colloidal properties of the latex particles is not disclosed. In U.S. Pat. No. 4,983,488, there is illustrated a process for the preparation of toners by the polymerization of a polymerizable monomer dispersed by emulsification in the presence of a colorant and/or a magnetic powder to prepare a principal resin component, and then effecting coagulation of the resulting polymerization liquid in such a manner that the particles in the liquid after coagulation have diameters suitable for a toner. It is indicated in column 9 of this patent that coagulated particles of 1 to 100, and particularly 3 to 70 are obtained. This process is thus primarily directed to the use of coagulants, such as inorganic magnesium sulfate which results in the formation of particles with wide GSD. Similarly, the aforementioned disadvantages are noted in other prior art, such as U.S. Pat. No. 4,797,339, wherein there is disclosed a process for the preparation of toners by resin emulsion polymerization, wherein similar to the '127 patent polar resins of opposite charges are selected, and wherein flocculation is not disclosed; and U.S. Pat. No. 4,558,108, wherein there is disclosed a process for the preparation of a copolymer of styrene and butadiene by specific suspension polymerization. Other patents mentioned are U.S. Pat. Nos. 3,674,736; 4,137,188 and 5,066,560.
In U.S. Pat. No. 5,290,654, the disclosure of which is totally incorporated herein by reference, there is disclosed a process for the preparation of toners comprised of dispersing a polymer solution comprised of an organic solvent and a polyester, and homogenizing and heating the mixture to remove the solvent and thereby form toner composites. Additionally, there is disclosed in U.S. Pat. No. 5,278,020, the disclosure of which is totally incorporated herein by reference, a process for the preparation of in situ toners comprising a halogenization procedure which, for example, chlorinates the outer surface of the toner and results in enhanced blocking properties.
In U.S. Pat. No. 5,308,734, the disclosure of which is totally incorporated herein by reference, there is illustrated a process for the preparation of toner compositions which comprises generating an aqueous dispersion of toner fines, ionic surfactant and nonionic surfactant, adding thereto a counterionic surfactant with a polarity opposite to that of said ionic surfactant, homogenizing and stirring said mixture, and heating to provide for coalescence of said toner fine particles.
In U.S. Pat. No. 5,346,797, the disclosure of which is totally incorporated herein by reference, there is disclosed a process for the preparation of toner compositions comprising
(i) preparing a pigment dispersion in a water, which dispersion is comprised of a pigment, an ionic surfactant, and optionally a charge control agent; PA1 (ii) shearing the pigment dispersion with a latex mixture comprised of a counterionic surfactant with a charge polarity of opposite sign to that of said ionic surfactant, a nonionic surfactant and resin particles, thereby causing a flocculation or heterocoagulation of the formed particles of pigment, resin and charge control agent to form electrostatically bounded toner size aggregates; and PA1 (iii) heating the statically bound aggregated particles to form said toner composition comprised of polymeric resin, pigment and optionally a charge control agent.
Also, a number of copending applications illustrate various emulsion/aggregation processes for the preparation of toners, such as U.S. Pat. Nos. 5,344,738; 5,403,693; 5,418,108; and 5,364,729, the disclosures of which are totally incorporated herein by reference. In U.S. Pat. No. 5,403,693, there is illustrated an emulsion-aggregation process where during the process there is added further anionic or nonionic surfactant in the range of from about 0.1 to about 10 percent by weight of water to control, prevent, or minimize further growth or enlargement of the particles in the coalescence step. The present patent application teaches the use of termination agents in the preparation of polymer particles by emulsion polymerization in a manner that reduces the tendency of the aggregates formed from such particles to break apart in, for example, the emulsion/aggregation process described in U.S. Pat. No. 5,403,693.